This invention relates to driving wheel slip control systems, and more particularly to improvements in or to such systems, which enable proper slip control when the driving wheels are in a state wherein the slip has a magnitude smaller than that in an excessive slip state.
As recognized in general, a driving wheel of an automotive vehicle undergoes a slip when the vehicle is started to run or when it is accelerated, if the driving force of the driving wheel surpasses a frictional force developed between the tire of the driving wheel and the road surface [=the coefficient of friction between the tire and the road surface.times.load of the vehicle weight on the driving wheel (wheel load)]. The frictional force acts in the advancing or longitudinal direction of the vehicle. The magnitude of the slip may be represented by a slip rate .lambda. which is expressed by the following equation: EQU .lambda.=(V.sub.W -V)/V.sub.W I
where V.sub.W represents the circumferential velocity of the driving wheel, and V represents the speed of the vehicle.
The frictional force F between the driving wheel tire and the road surface, which defines the upper limit of the effective driving force of the driving wheel, varies with the slip rate .lambda., as shown in FIG. 1. It will be noted from the figure that the frictional force F assumes the maximum value when the slip rate .lambda. has a predetermined value .lambda..sub.0. While the longitudinal frictional force F varies with slip rate .lambda. as indicated by the solid line in the graph, the transverse frictional force, which acts in the transverse direction of the vehicle, varies with slip rate .lambda., as indicated by the broken line in the graph, such that it becomes smaller as the slip rate increases.
A slip prevention system has been proposed, e.g. by Japanese Patent Publication (Kokoku) No. 52-35837, which is based upon the above recognition of the relationship between longitudinal frictional force, transverse frictional force and slip rate, and which controls the slip rate such that the longitudinal frictional force may be the maximum so as to obtain the maximum driving efficiency of the vehicle, while the transverse frictional force may have a drop as small as possible so as to prevent a skid or sideslip of the vehicle.
The above proposed control system is constructed such that the slip of the driving wheels is prevented from becoming excessive through control of the torque of the engine by turning on and off an ignition device of the engine of the vehicle or by allowing and inhibiting fuel supply from a fuel supply device to the engine.
Subsequently, a slip control system has also been proposed, e.g. by Japanese Provisional Patent Publication (Kokai) NO. 60-151,131, which also allows and inhibits the fuel supply for slip control, but is adapted such that when there occurs an abnormal rise in the temperature of a catalytic converter arranged in the exhaust system of the engine, the air-fuel ratio of a mixture supplied to the engine is controlled to a leaner value or to a richer value than a normal value required by the slip state, thereby preventing what is called after-fire, i.e. the phenomenon that unburnt fuel burns in the engine exhaust system, or preventing burning of a catalyst device disposed in the exhaust system for purging toxic components in the exhaust gases.
However, the above proposed fuel supply control does not contemplate the influence of engine rotational speed upon required fuel characteristics of the engine. As a result, the proposed control system can still undergo after-fire or degraded driveability of the engine at low engine rotational speeds, while the catalytic converter is still apt to rise in temperature and even burn to be damaged at high engine rotational speeds.
Furthermore, according to the prior art no contemplation has been made of load on the engine in effecting the fuel supply control for slip control. For instance, in the case of controlling the air-fuel ratio to a leaner value in the above-mentioned specific lower slip state, the amount of drop in the torque or driving force of an internal combustion engine to be caused by reducing the fuel amount supplied to the engine varies depending upon the magnitude of load on the engine. Therefore, in effecting slip control independently of load on the engine in the specific lower slip state, if the target air-fuel ratio is set at a value conforming to a high load operating condition of the engine, slip control is effected to an excessive extent when the engine is operating in a low load operating condition, whereas if the target air-fuel ratio is set at a value conforming to a low load operating condition of the engine, slip control is effected to an insufficient extent when the engine is operating in a high load operating condition. This, therefore, makes it impossible to achieve desired values of slip rate over the entire load range, thereby failing to secure good driveability of the vehicle.